Although patient data was not accessed, the pathology system at London’s Barts Health was offline for a few days.

A previously unseen malware is being blamed for an attack on a London hospital that forced the facility to shut down a segment of its systems for a few days as a precautionary measure.

Barts Health NHS Trust, a conglomerate of five hospitals in London employing a staff of 15,000, was hit in January by the malware attack, which managed to circumvent the facility’s anti-virus software, according to a report on ZDNet.

Although administrators at Barts Health said patient data was not accessed, the facility’s pathology system was offline for a few days.

While previous reports said it was unclear how the trojan had gotten into the hospital’s network, an update from March 1 board minutes revealed that four of the hospital group’s five facilities were affected. While the hospital’s AV software was up to date, the incident involved “a new virus not seen previously.”